Fraternal Birth Order Effect and Other Sibship Effects in Bisexual and Asexual Men and Women.

Previous research has shown that homosexual men and women tend to have more older brothers than their heterosexual counterparts-this is known as the fraternal birth order effect (FBOE) and is usually considered a marker for biological development of sexual orientation. Recent studies suggest this effect might also apply to bisexual and asexual individuals who have been overlooked by previous scholarship. In this study, I explored associations of the FBOE and other sibship effects with minority sexual orientation in an online collected sample of heterosexual (n = 1174), bisexual (n = 432), and asexual (n = 90) men and women from Czechia and Slovakia. Results of the binary logistic regression indicated the FBOE in bisexual men (OR = 1.66) but not in other sexual minority groups. There were no significant differences in other sibship characteristics between sexual minority and heterosexual individuals, although other non-heterosexual study groups showed a non-significant FBOE of a magnitude similar to bisexual men. This study contributes to the limited literature on understudied bisexual and asexual populations and indicates that male bisexuality might share a common biodevelopmental origin with male and possibly also female homosexuality.